We measure distance, time, speed, mass, pressure, temperature, volume… — Page 1, Lesson 8

We are starting a brand-new chapter, Chapter 1, called "Characteristics and General Definitions." This is the foundation of the entire instrumentation syllabus, so I want to set the tone properly. Before we talk about any instruments, we have to agree on the language we use to describe the physical world around the aircraft. That language is units. Let me walk you through the units table, because this is the vocabulary you will use every single day as a professional pilot. We measure distance, time, speed, mass, pressure, temperature, volume, angles, and position. And in aviation, we don't just use one system—we use several, and you must know how they relate. Starting with distance. We have metres and kilometres, which are the metric standard. Then we have nautical miles, and this is the big one for aviation. One nautical mile equals 6080 feet, or 1852 metres. Remember that number—1852 metres—because it is the basis of all navigation. Then we have feet, which we use for altitude. And statute miles, the road mile, where one statute mile equals 5280 feet or 1609 metres. So you see the contrast: a nautical mile is longer than a statute mile. Time is straightforward—hours, minutes, seconds. Speed is where it gets interesting. We measure speed in knots, which is nautical miles per hour. We also have miles per hour and kilometres per hour. A knot is not the same as a mile per hour, because it is based on the nautical mile. Mass is kilogram and pound. Pressure is a big one, and I want you to pay close attention here because this is where the units get dense. We measure pressure in pascals and hectopascals. One standard atmosphere equals 1013.25 hectopascals. We also use bar and millibar—one atmosphere equals 1013.25 millibars. Notice that hectopascals and millibars are numerically the same value, 1013.25, which is why you will see them used interchangeably. Then we have inches of mercury—one atmosphere equals 29.92 inches of mercury. Millimetres of mercury—one atmosphere equals 760 millimetres of mercury. And pounds per square inch—one atmosphere equals 14.7 psi. So all of these are just different rulers measuring the same standard atmospheric pressure. Temperature. We use Celsius, and at mean sea level, standard temperature is plus 15 degrees Celsius. We use Kelvin, which is absolute temperature, and at mean sea level that is 288 Kelvin, sometimes written as 288 degrees absolute. And Fahrenheit, where mean sea level is plus 59 degrees Fahrenheit. Volume. Litres, pints, gallons. And here is a critical distinction: imperial and US gallons are not the same. One imperial gallon equals 1.2 US gallons. That is a fuel-loading trap if you ever mix them up. Angles. Degrees, minutes, seconds. And position—latitude and longitude—which are shown in degrees, minutes, and seconds. So the takeaway from this table is that aviation is a multi-unit environment. You must know the exact conversion factors, especially the nautical mile, the hectopascal, and the imperial versus US gallon, because these are the ones that cause real-world errors. That is our foundation. Now we are ready to move into the introduction and the concept of measuring range versus accuracy.